Most pharmaceutical formulations are available in salt forms. In fact, many formulations are only available in the form of a pharmaceutically acceptable salt. Salts have long been considered advantageous because of their high stability, ease of handling and formulation and generally high water solubility. Unfortunately, salt formulations do not tend to be useful in transdermal drug delivery systems. Because of the growing acceptance of such drug delivery systems by the general public, the inability to conveniently produce transdermal patches utilizing various pharmaceutical formulations is a great disadvantage.
There are many possible explanations for the general incompatibility of salt forms of drugs and transdermal performance. For example, protonated pharmaceutically active compounds (basic salts) are generally relatively high in polarity. It is known, however, that non-polar drugs, in general, are transmitted through the skin easily because of a high degree of compatibility with lipophillic layers of the skin. Highly polar substances such as salt forms of drugs and indeed some free forms of drugs, by virtue of their incompatibility with such lipophillic layers, are generally very slow in permeating skin.
One approach to forming a transdermal patch to overcome such problems was suggested in Yoshida et al., U.S. Pat. No. 4,738,848 and Nakano et al., U.S. Pat. No. 4,740,374. According to these patents, compounds such as diclofenac sodium and non-steroidal anti-inflammatory analgesic agents, when present in their salt forms, are difficult to dissolve into a pressure sensitive adhesive material having relatively high lipophillic properties. It is also difficult to maintain the active ingredient therein. If large amounts of drugs are added to the adhesive, in some cases, the drug cannot be dissolved or crystallization of the drug may occur. This makes it impossible to deliver a sufficient amount of the drug into the skin.
According to Yoshida et al. and Nakano et al., these difficulties can be overcome by concurrently using an organic acid during the formulation of the adhesive material. The organic acid apparently increases the solubility of the active ingredient in the pressure sensitive adhesive material and also increases the percutaneous absorption properties thereof. These references express their belief that the reason for the increased absorption properties is that the drug is converted to its free form having a higher oleophilicity (lipophilicity) resulting in the higher solubility of the drug.
Another approach was taken in Heiber et al. U.S. Pat. No. 4,917,676, which relates to a user-activated transdermal therapeutic system. The transdermal drug delivery system described therein includes separate compartments for various formulations in “pre-activated states”. Just prior to use, the patient or other person applying the system allows the partitioned ingredients to commingle, thus activating the system. The user generally bursts a burstable barrier separating the two reservoirs. Then the therapeutic agent, usually in the form which must be altered for the desired transdermal delivery, and the activating substance combine and transform the therapeutic agent to a suitable species.
Inactive forms of therapeutic agents in accordance with Heiber et al. can include, for example, an acidic drug which, as an ionized species, penetrates skin to a slight degree, but in a free acid form, permeates freely through the skin. Activating substances may include pH regulators such as buffers, acids or bases.
Such transdermal systems, however, suffer from several disadvantages. First, they require a rather complex arrangement of two or more compartments separated by, for example, a burstable but otherwise nonpermeable material. In addition to the complexity of such a structure and the potential difficulties in separately filling and maintaining discretely the individual compartments, there is also the problem of premature bursting of the burstable layer and the premature intermingling of the various components. Clearly, the ability to manufacture a transdermal device wherein all of the necessary ingredients can be intermixed and intermingled from the start and added together to each and every cavity in a transdermal patch would be a great advantage. Finally, the Heiber et al. patent considers the complexities of forming a patch where the therapeutic and activity agents are mixed and maintained together at the time of manufacture or the subsequent storage stability problems attendant such a mixture.
Moreover, resolving the question of the physical state of a drug does not resolve all of the issues surrounding the production of transdermal patches from certain highly plasticizing drugs. In fact. providing these drugs in a free base form could actually raise additional problems. It comes as no surprise that a drug or solvent loaded into an adhesive system will have an effect on the adhesive properties of the resulting mixtures. In certain cases, with certain drugs, the effect on the hardness and tackiness of the resulting adhesive mixture is minimal. However, in certain other instances, drugs such as, for example, nitroglycerin or nicotine may act as plasticizers for many conventional adhesive systems. Plasticizing drugs such as these, can have a significant deleterious effect on the physical properties of the resulting adhesive matrix depending upon the type of drug, and the amount used. Generally, plasticizing drugs act to soften or disturb the structural integrity of the adhesive making it more fluid like and can, in certain cases, negatively effect the degree of adhesivity.
A number of companies have introduced either high molecular weight or highly crosslinked adhesive systems. It is known that these systems can generally be used almost interchangeably with plasticizing drugs. Typical examples of such adhesives include, without limitation, GELVA 737, GELVA 2655, and GELVA 1753 self crosslinkable acrylic adhesives from Monsanto's Chemical Group, 730 Worcester Street, Springfield, Mass. 01151 and DUROTAK 87-2516, DUROTAK 87-2194 and DUROTAK 87-2852 self crosslinkable acrylic adhesives available from National Starch and Chemical Company, 10 Finderne Ave., P.O. Box 6500, Bridgewater, N.J. 08807-0500. All of these crosslinked adhesives find wide spread use in the pharmaceutical industry in the formulation of transdermal drug delivery systems. When liquid, lipophillic drugs are added to these adhesives at amounts of between 30 and about 40% the resulting material would generally not suffer deterioration in physical properties so as to render many of these acrylic based adhesives unusable. While many of these adhesives are virtually interchangeable, of course, some combinations of a specific drug and a specific adhesive may provide marginally better properties.
When the inventors attempted to construct a transdermal delivery vehicle for selegiline, a particularly highly plasticizing drug, they too expected that selegiline patches produced with any of the foregoing class of adhesives could be accomplished without a problem. This was particularly true as loading levels were anticipated at only between about 10 and about 20%; not particularly challenging for these adhesives.
As illustrated in Table 1, when mixtures of selegiline (15 wt %) and various adhesive materials were tested using conventional performance tests, they all demonstrated comparable and generally acceptable results.
TABLE 1POLKEN TACK OF VARIOUSTRANSDERMAL ADHESIVES WITH 15% SELEGILINEADHESIVEPOLKEN TACKGELVA 1753346DUROTAK 87-2194453GELVA 737333DUROTAK 87-2516286
Yet when these formulations were tried on skin, the results were quite surprising. While some of the formulations worked, others unexplainably exhibited significant cohesive failure whereby adhesive remained on the skin after a transdermal patch was peeled-away. The disparity in the results obtained between conventional “bench-top” testing and actual field application was truly discouraging. It essentially placed a whole host of established tests in a highly compromised state.
The inventors were also taken aback by the degree of disparity observed. When they formulated a selegiline containing transdermal patch with, for example, DUROTAK 87-2194, those patches exhibited cohesive failure and adhesive transfer. Formulations made with GELVA 2655 exhibited total adhesive failure. Neither result could have been predicted based on results such at that reported in Table 1. This problem was only amplified by the use of other traditional tests such as a measure of shear strength. As shown in Table 1A, a number of formulations including selegiline were measured in terms of shear strength.
TABLE 1ASHEAR STRENGTH OF DIFFERENT ADHESIVE SYSTEMS WITHSELEGILINE BASEADHESIVESHEAR (MIN)SELEGILINEGELVA 7374.3113% GELVA 7883.113% DUROTAK 87-2516117413 %DUROTAK 87-21943613 %GELVA 175314404%GELVA 175314408%GELVA 1753144013 %
Typically, shear values of greater than about a half an hour to one hour would be considered acceptable adhesive systems. As one can see from Table 1A, GELVA 1753 produced relatively high shear rates, which should indicate an acceptable adhesive system. However, DUROTAK 87-2516 also exhibited acceptable shear, and the formulations made from this adhesive were totally unacceptable when applied to skin. In addition, selegiline with 10% propylene glycol as a solvent, provided shear values of greater than 800 minutes when formulated with GELVA 1753. However, while such results are generally indicative of good adhesion characteristics, this particular formulation exhibited very poor adhesion.
Much to their dismay, the inventors discovered that with a certain class of particularly highly plasticizing drugs, only selected adhesives would work. They also found that, based on the state of the art, they could not predict which adhesives would work and which would not.